Genetics is the study of heredity. If you understand genetics you will be able to work out why you look like each of your parents and your siblings (brothers and sisters) but are not exactly like any one of them: unless you have an identical twin.

Don’t be worried in the exam if you are given an example which you have not studied: if you understand the principles you will probably find GCSE question on genetics quite easy.

Don’t you just hate it when you are lying in your pram gurgling away and all your dad’s family come up and say “Ooooh doesn’t he look just like his dad?” and then all your mum’s family come and say “Oooh doesn’t she look just like her mum?”. None of them studied genetics!


  1. DNA
  2. Genes and Alleles
  3. Chromosomes
  4. Inheritance of Sex
  5. Blue and Brown eyes (other colours as well)
  6. PKU
  7. Albinism
  8. Haemophilia
  9. Sickle Cell
  10. How you inherit your Blood Group
  11. Glossary

All living things inherit their characteristics in very much the same way. What you learn here is true for all animals, green plants and fungi. There a some differences in the mechanisms in prokaryotes even though they use DNA as we do. This page is just about inheritance in Eukaryotes, i.e. those things with proper nuclei. (animals, green plants and fungi.)


DNA stand for Deoxyribo Nucleic Acid; you don’t need to remember what DNA stands for in your GCSE exam, but should should know that DNA is a very special chemical which carries the hereditary information in almost all living things. (Some viruses use RNA instead of DNA.) DNA is special because it carries the genetic code which determines how cells make proteins AND it can be replicated exactly.

So when a cell divides, it begins by duplicating all its DNA. This DNA is then divided between the two daughter cells produced in mitosis. We all started our lives inside our mums as a single fertilised egg or zygote. Within a few hours of conception we started to divide and divide again and again into millions of cells. Because these divisions were genetically exact, each one of us consists of billions of genetically identical cells. Siblings (brothers and sisters) are normally created from totally different eggs and sperms and so whilst we are similar to our siblings we are not identical to them. Very occasionally a single fertilised egg separates into two separate cells which each become a developing baby. In this case the two babies will be identical twins: you will discover as you read on why identical twins MUST be the same sex (two girls or two boys) and ordinary twins could be the same sex or different sexes.

Genes and Alleles

The nuclei in our cells are not test tubes containing lots of chemicals all mixed together. In fact there is a lot of DNA inside every nucleus but it is all carefully organised. A human cell contains millions of bits of DNA, each bit is a piece of information which is used to control how the cell works and ultimately what we look like. My hair is brown (well it is supposed to be brown) because that is what the information in my genes make it (this is my genotype). If you saw me you would see that it is now going grey, that is because of all the stress that I have as a teacher and because I am getting older; this is not to do with my genes but is caused by my environment (what you see is my phenotype). For every characteristic a cell contains two bits of DNA or genes. These two genes may be the same as each other (homozygous) or different to each other (heterozygous).

There are three possible combinations that you might inherit: two dominant alleles, two recessive alleles, or one dominant and one recessive allele. e.g. I know that I have two different alleles for eye colour. I have brown eyes because I inherited a brown eye allele from my father who also had brown eyes, but I have also inherited a blue eye allele from my mother who has blue eyes. Because the brown eye allele is dominant my eyes are brown.


Human cells normally contain 46 chromosomes. A chromosome is a thread of protein with many genes attached to it. I prefer to think of these chromosomes as 23 pairs rather than as 46 separate ones; this is because each one of us inherits 23 chromosomes from each parent. When we make gametes (sperms and eggs) a different kind of cell division occurs. In the testis and ovary cells divide by another kind of cell division called meiosis in which the number of chromosomes is reduced from 46 to 23. When a sperm fertilises an egg, there will be 23 pairs of chromosomes again. Of these 23 pairs of chromosomes, 22 pairs control quite ordinary features like eye colour and hair colour. One pair of chromosomes controls sex i.e. which one you will be. These are the X and Y chromosomes.

Inheritance of Sex

Girls/women always carry two X chromosomes in every cell of their bodies. When they make eggs by meiosis the number of chromosomes is cut in half; so eggs only contain one X chromosome.

Boys/men only have one X chromosome, but they also have a Y chromosome in every cell. When they make sperms, some of the sperms carry an X chromosome and some carry the Y chromosome instead. XX make you female, XY makes you male.

The X chromosome carries some genes which have very little if anything to do with the inheritance of sex. These characteristics, e.g. haemophilia, colour blindness, are sex linked: in other words the inheritance of these genes is linked to the inheritance of sex because they are carried on the sex chromosomes. The Y chromosome is very small and carries very few genes. The genes for sex linked characteristics like haemophilia have an interesting pattern of inheritance. Women have two genes for normal blood clotting whereas men only have one gene for blood clotting on the X chromosome (the Y chromosome does not have this gene.)

Blue and Brown Eyes

The allele for brown eyes is dominant, and the allele for blue eyes is recessive. I use the symbol B for the brown eye allele and b for the blue allele. There are three possibilities: BB, Bb and bb. These are the three possible genotypes. If you inherit either BB or Bb then you will have brown eyes. If you inherit bb you will have blue eyes. (I am counting brown, black and hazel eyes as being brown and blue, grey and green as being blue: other genes determine what shade of blue or brown your eyes are.) The two possible phenotypes are blue eyes and brown eyes.


This is a genetic disease in which the liver is unable to remove the chemical phenyl ketone which ends up being carried around the body in the blood and excreted in the urine: it is caused by a recessive gene. Phenyl ketone is a solvent and it causes damage to nerve cells. (to be updated ASAP.) PKU is passed on from generation to generation by the same simple mechanism as eye colour. Approximately 1 person in 80 carries the mutant (defective or changed) gene for PKU. The chances of two people both carrying this mutant gene are 1 in 6400; and the chance of them having a baby with PKU is about 1 in 25600 so there is no need to worry about it unless there is a history of this disease in your family.


Albinism is caused by a mutant gene carried by about 1 person in 1000. The chance of two people both carrying the gene is about 1 in a million; and of a baby being born with two mutant genes is 1 in 4 million. I have seen 2 albinos in London. Albino means white. Someone who inherit two recessive albino genes cannot make the black pigment called melanin. Most of us make melanin in our skin, hair and at the back of our eyes. (to be updated ASAP.) Again, the pattern of inheritance is a simple one just as for PKU and eye colour.


The inheritance of this genetic disease is a bit more complicated because it is sex linked. Women normally have two genes for blood clotting, one on each X chromosome. Men only have one blood clotting gene because they only have one X chromosome. This gene is not present on a Y chromosome. Some women carry a defective gene for haemophilia. These women are called carriers. Their daughters may be normal or may be carriers. Their sons are either normal or actually have the disease. A haemophiliac male has problems when blood vessels are damaged because his blood will not clot properly. He can pass the gene on to his daughters who will be carriers. ALL his daughters will be carriers because the defective gene is on the X chromosome. There is nothing wrong with his Y chromosomes, so his sons will all be perfectly normal.

Some of Queen Victoria’s descendants inherited this gene from her. She must have been a carrier. If you read about the Russian Revolution, you will find out about Tsar Nicholas and his family getting murdered by the Bolsheviks. The Tsar’s son Alexis (the Tsarevitch) did not die of haemophilia! He suffered terribly from haemophilia and his mother (the Tsarina) asked the monk Rasputin to help cure her son. However anyone would die if the were filled up with bullet holes. The story of The Tsar and his family is a very interesting one. Ask your history teacher who inherited the Romanov family fortune.

Sickle Cell

The inheritance of this genetic disease is a bit more interesting because it can be an advantage to have the gene as well as being a disadvantage. In this case there are three possible phenotypes instead of two. Most people produce normal haemoglobin (this is the protein found in red blood cells which carries oxygen around the body). Sickle cell haemoglobin is very slightly different. Sickle cell haemoglobin works just as well as ordinary haemoglobin unless the Oxygen level in the blood goes down to zero; this happens when someone is running in a race and builds up an oxygen debt. When you have an Oxygen debt your muscles start to make lactic acid. Sickle cell red blood cells collapse when they have no Oxygen; this can result in death!!!

So ordinary people have two ordinary Haemoglobin genes HH. Some people have two sickle cell genes SS; they have the full sickle cell anaemia. Neither the H nor the S allele is dominant; so biologists just have to give it a special name: they call this co-dominance. People who inherit both alleles, HS, have sickle cell trait. Their red blood cells contain both kinds of haemoglobin.

Obviously if you have a gene which might kill you, you would consider it to be a very big disadvantage! However, if you live in an area of the world where malaria is a common disease there is an advantage in having the sickle cell gene. When the malaria parasite gets into red blood cells they quickly use up all the Oxygen in them; this results in the red blood cells collapsing; this happens before the malaria parasite can reproduce.

Why do people make such a fuss about HIV and AIDS? It is malaria which kills the most people in the world every year. Having sickle cell can be a very big advantage; you can let the mosquitoes bite you without getting malaria. Providing that you do not get overstressed and build up an Oxygen debt, you will live happily ever after.



DNA: this is the chemical which carries genetic information in the nuclei of cells. back

Gene: this is a bit of DNA. back

Chromosome: this is a strand of protein with a large number of DNA molecules attached to it. The number of chromosomes varies between different animals and plants. The full number in humans is 46 (23 pairs). back

Alleles: these are different versions of a gene e.g. B and b see eye colour above. back

Dominant: this is the allele which will be “expressed” in the phenotype if it is present as either a single gene or as two genes. e.g. B. back

Recessive: this is the allele which is only expressed if these are two such alleles e.g. b. back

Co-dominance: this is when neither of two alleles is dominant to the other. Here there are three genotypes HH, HS and SS; each produces a different phenotype. back

Homozygous: this means having two identical alleles, e.g. BB or bb. back

Heterozygous: this means having a dominant and a recessive allele e.g. Bb. back

Genotype: this is what genes you carry e.g. BB, Bb or bb. back

Phenotype: this is what you look like e.g. brown or blue eyes. back

Mitosis: this is a genetically exact division of a cell. back

Meiosis: this is a genetically inexact division of cell reducing the number of chromosomes to a half the normal number. This is what happens in the testis and ovary; meiosis produces gametes. back

Gametes: these are the sex cells i.e. sperms and eggs. Eggs are also called ova. back

Mutations: this is change or mistake in copying a gene. Most mutations result in recessive genes. The information in the DNA molecule has been corrupted and it will no longer produce the correct protein. back